Speed Converter (km/h, mph, knots)
Convert between km/h, mph, m/s, knots and ft/s instantly — all units shown at once, with exact factors.
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How It Works
Type a number into any field — km/h, mph, m/s, knots, or ft/s — and the other four update instantly, computed from exact unit definitions rather than rounded rule-of-thumb multipliers. Two conversion factors do all the work: 1 mph = 1.609344 km/h (from the international mile) and 1 knot = 1.852 km/h (from the nautical mile).
Those two factors are not the same distance, which is the detail most converters gloss over. The statute mile (1609.344 m) comes from old English land-surveying units. The nautical mile (1852 m) is defined differently: it's the length of one minute of arc along a great circle of the Earth — historically, one sixtieth of a degree of latitude. Because the Earth's circumference works out to almost exactly 21,600 nautical miles (360° × 60 minutes), sailors and pilots get a distance unit that lines up directly with the latitude scale on a nautical chart.
Worked example: a 70 mph US freeway speed converts to 112.65 km/h, 31.29 m/s, and 60.83 knots. A 20-knot wind is 37.04 km/h. A 10 m/s sprint pace is exactly 36 km/h, since m/s to km/h is just ×3.6.
What to Know
- "Knot" has a literal origin. Before mechanical logs existed, sailors threw a wooden board tied to a rope with evenly spaced knots off the stern and counted how many knots paid out in a fixed time, measured with a sandglass — hence "knots" became the unit itself, not just a count of them.
- Statute miles and nautical miles are not interchangeable. Road speed limits and car speedometers use the statute mile (mph); aviation and marine navigation use the nautical mile (knots). Mixing them up when reading a chart or a pilot's briefing is a genuine navigation hazard, not just a rounding error.
- All factors used here are exact, not approximated: the mile and nautical mile both have fixed legal definitions, so mph ↔ km/h and knot ↔ km/h conversions carry no error beyond floating-point display rounding.
- ft/s shows up mainly in US engineering and physics — muzzle velocities, terminal velocity, structural drop calculations — because it keeps everything in feet and seconds without switching to metric mid-calculation. It converts from m/s by the exact factor 1 m/s = 3.28084 ft/s, itself derived from the definition 1 foot = 0.3048 m exactly.
Frequently Asked Questions
Why do sailors and pilots use knots?
One knot equals one nautical mile per hour, and the nautical mile is defined as one minute of latitude arc — because Earth's circumference is almost exactly 21,600 nautical miles (360° × 60), a vessel's speed in knots translates directly to how fast it crosses degrees of latitude on a chart.
Are the conversion factors exact?
Yes. The international mile is defined as exactly 1.609344 km and the nautical mile as exactly 1852 m, so mph ↔ km/h and knot ↔ km/h conversions are exact by definition — the only rounding happens when the result is displayed.
Where does the word "knot" actually come from?
From the 16th-18th century practice of measuring a ship's speed with a "chip log": a wooden board attached to a rope with knots tied at regular intervals was thrown overboard, and sailors counted how many knots ran out while a sandglass emptied. The count in knots became the speed itself, and the name stuck even after mechanical and satellite logs replaced the rope.
How do I convert a running pace like min/km?
Pace is time per distance — the inverse of speed. Use our Running Pace Calculator for min/km ↔ min/mi; this tool covers distance-per-time units.
Why is 1 m/s exactly 3.6 km/h?
Because 1 m/s means 1 meter every second, and there are 3600 seconds in an hour and 1000 meters in a kilometer: 3600 ÷ 1000 = 3.6. Multiplying any m/s value by 3.6 gives km/h exactly, with no rounding involved — the same logic in reverse (÷3.6) turns km/h back into m/s.
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Speed Converter (km/h, mph, knots)
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